Fuel cell systems use a fuel and an oxidant to react to produce electricity in a fuel cell stack. One type of fuel cell system employs a proton exchange membrane (hereinafter “PEM”). The PEM is a solid polymer electrolyte membrane that facilitates transfer of protons from an anode to a cathode in each individual fuel cell normally deployed in a fuel cell system. The electrodes and membrane together form a membrane electrode assembly (MEA). The electrodes contain catalysts to catalytically facilitate reaction of the fuel (such as hydrogen) and the oxidant (such as oxygen or air) to generate the electricity.
In a typical PEM fuel cell, the MEA is disposed between gas diffusion media (GDM). The GDM and MEA are disposed between a pair of electrically conductive plates. If the plates are bipolar plates, the plates conduct current between adjacent fuel cells in the fuel cell system. If the plates are unipolar plates at an end of the fuel cell system, the plates conduct current externally of the fuel cells.
As described in commonly-owned, co-pending U.S. patent application Ser. No. 11/762,845, hereby incorporated herein by reference in its entirety, the goal of an anode supply manifold purge operation is to completely fill the anode supply manifold with hydrogen. The anode supply manifold is filled with hydrogen prior to filling active areas of the anode with hydrogen during startup. Generally, the anode supply manifold is filled with hydrogen by opening a manifold purge valve at the top of the anode supply manifold while producing a flow of hydrogen into the bottom of the anode supply manifold. Purge valves may allow emission of the hydrogen flowing therethrough to the atmosphere, thereby increasing a cost to operate and an amount of time to startup, the fuel cell system.
A flow rate of hydrogen, along with a flow resistance of the purge valve, creates a back pressure in the anode supply. Hydrogen is then caused to uniformly flow through the active area. During this time, electrode to electrolyte potential differences on the cathode in the air/air part of the cell can be quite high (higher than the measured cell voltage) as driven by the hydrogen/air part of the cell. Stack loads can be used to suppress the cell voltages. To limit the cumulative effect of these events where hydrogen fills the anode of an air filled stack, it is desired to quickly fill the anode. The gas resident within the anode is discharged from the system through a discharge valve to the exhaust. However, the anode flush rate of air is limited by the pressure limit of the anode and the size of this discharge valve. This discharge valve must be closed before hydrogen is discharged to avoid hydrogen emissions concerns or the rate of discharge must be further reduced such that dilution air in the exhaust can bring any discharged hydrogen to an acceptable mole fraction.
It would be desirable to develop a method for filling an active area of an anode side of each fuel cell of a fuel cell system with hydrogen prior to a start-up operation, wherein the anode is quickly filled with hydrogen and discharge of hydrogen from the system is militated against.